1. Field of the Invention
This invention relates to a fuel vapor capturing canister for capturing fuel vapor produced in a fuel system of an engine by means of an adsorbent layer to prevent dissipation of the fuel vapor into the atmosphere.
2. Description of the Prior Art
Japanese Unexamined Patent Application Publication No. 50-22921 (1975) discloses a conventional canister of the above-described type. Referring to FIG. 10, a sectional view of the disclosed canister is shown. A cylindrical casing 1 is made of an impermeable material and accommodates an activated carbon layer 2 therein. One of two surfaces of the activated carbon layer 2 communicates with the atmosphere through an opening 3a of a retainer 3. The other surface of the activated carbon layer 2 communicates both with a fuel tank 4 and with an intake pipe 5 through separate openings 1a and 1b, respectively. The activated carbon layer 2 is divided into two layers 2a and 2b axially of the casing 1 by an elastic member 6 comprised of a plate-shaped permeable member.
In the above-described construction, fuel vapor produced in the fuel tank 4 enters the upper activated carbon layer 2a through the opening 1a. Passing through the permeable elastic member 6, the fuel vapor then enters the lower activated carbon layer 2b. The fuel component is thus captured by the activated carbon as the fuel vapor passes through its layer. On the other hand, upon start of an engine, a negative pressure is supplied to the opening 1b communicating with the intake pipe 5. Consequently, air is drawn through the lower activated carbon layer 2b, the elastic member 6 and the upper activated carbon layer 2a sequentially, whereupon the fuel component captured by the activated carbon is purged therefrom to be supplied through the intake pipe 5 to the engine.
In the above-described canister, the fuel vapor is adsorbed by, and purged from the, activated carbon as the air containing it flows axially of the casing 1. Accordingly, the distance that the air containing the fuel vapor passes through the activated carbon layer 2 depends upon the length of the casing 1. The fuel vapor capturing efficiency is lowered as the casing 1 becomes shorter. Thus, the above-described canister poses a problem of the fuel vapor capturing efficiency.
Furthermore, the prior art has proposed another construction in which the air containing the fuel vapor is caused to reciprocally flow along complicate flow paths in the casing 1. However, only part of the activated carbon serves to capture the fuel vapor in each flow path, and accordingly, an entire activated carbon accommodated in the casing 1 cannot be used efficiently.